Plugin idea - promotion with gift

[gabiudrescu]

Based on Sylius/Sylius#1406

If someone is interested in this, maybe it should be ported to 1.0.

[bitbager]

Leaving it as opened. Maybe we will prepare some new section for plugin ideas so that people who want to learn more about plugin development will have a nice place to start. I also know that Sylius is planning to encourage more people to write new plugins. This might be an interesting aspect.